Bottom Line:
Remarkably, tumor tissue exhibited a decreased prevalence of these mutations relative to adjacent non-tumor tissue.The difference in mutation burden was attributable to a reduction in C:G to T:A transitions, which are associated with oxidative damage.We demonstrate that the lower random mutation frequency in tumor tissue was also coupled with a shift in glucose metabolism from oxidative phosphorylation to anaerobic glycolysis, as compared to non-neoplastic colon.

ABSTRACTGenome instability is regarded as a hallmark of cancer. Human tumors frequently carry clonally expanded mutations in their mitochondrial DNA (mtDNA), some of which may drive cancer progression and metastasis. The high prevalence of clonal mutations in tumor mtDNA has commonly led to the assumption that the mitochondrial genome in cancer is genetically unstable, yet this hypothesis has not been experimentally tested. In this study, we directly measured the frequency of non-clonal (random) de novo single base substitutions in the mtDNA of human colorectal cancers. Remarkably, tumor tissue exhibited a decreased prevalence of these mutations relative to adjacent non-tumor tissue. The difference in mutation burden was attributable to a reduction in C:G to T:A transitions, which are associated with oxidative damage. We demonstrate that the lower random mutation frequency in tumor tissue was also coupled with a shift in glucose metabolism from oxidative phosphorylation to anaerobic glycolysis, as compared to non-neoplastic colon. Together these findings raise the intriguing possibility that fidelity of mitochondrial genome is, in fact, increased in cancer as a result of a decrease in reactive oxygen species-mediated mtDNA damage.

Mentions:
Next, we quantified the frequency of random mutations in colorectal tissue using the RMC assay. We found that in normal colorectal tissue, mutations occurred at an average absolute frequency of 3.2±0.5×10−5 per base pair at a site in the 12S rRNA subunit (Figure 1A), and 1.2±0.2×10−4 at a second site in the COXI gene (Figure 1B). At both sites, mutation frequency was independent of patient age (Figure 2A). Unexpectedly, when we examined patient-matched colorectal carcinomas we found that, on average, tumor cells displayed an approximately three-fold lower random mutation frequency compared to normal colonic tissue (1.4±0.4×10−5, P = 0.004 for 12S rRNA, 4.1±1.2×10−5, P<0.001 for COXI, 2 tailed paired t-test) (Figure 1A–1B). No appreciable difference in mtDNA copy number was observed between the tissues (Figure 2B). These mtDNA findings are in stark contrast to previous observations in which we demonstrated a>100-fold increase in the frequency of random mutations in the nuclear DNA of human tumors [11].

Mentions:
Next, we quantified the frequency of random mutations in colorectal tissue using the RMC assay. We found that in normal colorectal tissue, mutations occurred at an average absolute frequency of 3.2±0.5×10−5 per base pair at a site in the 12S rRNA subunit (Figure 1A), and 1.2±0.2×10−4 at a second site in the COXI gene (Figure 1B). At both sites, mutation frequency was independent of patient age (Figure 2A). Unexpectedly, when we examined patient-matched colorectal carcinomas we found that, on average, tumor cells displayed an approximately three-fold lower random mutation frequency compared to normal colonic tissue (1.4±0.4×10−5, P = 0.004 for 12S rRNA, 4.1±1.2×10−5, P<0.001 for COXI, 2 tailed paired t-test) (Figure 1A–1B). No appreciable difference in mtDNA copy number was observed between the tissues (Figure 2B). These mtDNA findings are in stark contrast to previous observations in which we demonstrated a>100-fold increase in the frequency of random mutations in the nuclear DNA of human tumors [11].

Bottom Line:
Remarkably, tumor tissue exhibited a decreased prevalence of these mutations relative to adjacent non-tumor tissue.The difference in mutation burden was attributable to a reduction in C:G to T:A transitions, which are associated with oxidative damage.We demonstrate that the lower random mutation frequency in tumor tissue was also coupled with a shift in glucose metabolism from oxidative phosphorylation to anaerobic glycolysis, as compared to non-neoplastic colon.

ABSTRACTGenome instability is regarded as a hallmark of cancer. Human tumors frequently carry clonally expanded mutations in their mitochondrial DNA (mtDNA), some of which may drive cancer progression and metastasis. The high prevalence of clonal mutations in tumor mtDNA has commonly led to the assumption that the mitochondrial genome in cancer is genetically unstable, yet this hypothesis has not been experimentally tested. In this study, we directly measured the frequency of non-clonal (random) de novo single base substitutions in the mtDNA of human colorectal cancers. Remarkably, tumor tissue exhibited a decreased prevalence of these mutations relative to adjacent non-tumor tissue. The difference in mutation burden was attributable to a reduction in C:G to T:A transitions, which are associated with oxidative damage. We demonstrate that the lower random mutation frequency in tumor tissue was also coupled with a shift in glucose metabolism from oxidative phosphorylation to anaerobic glycolysis, as compared to non-neoplastic colon. Together these findings raise the intriguing possibility that fidelity of mitochondrial genome is, in fact, increased in cancer as a result of a decrease in reactive oxygen species-mediated mtDNA damage.